Direct Access Storage Devices (DASD) usually take the form of a rotating data storage member, such as a magnetic disk, magnetooptic disk, and the like. In most data processing systems, such DASDs are a primary peripheral data storage for operating the data processing system. The format of the data on the DASD includes gaps between various fields of data and control data on such DASD. The arrangement has been such that control interaction between a controller or control unit of the peripheral subsystem and a host processor occurs while the transducer of the DASD is scanning a gap. In this manner, the operation of the host processor and of the peripheral subsystem are synchronized. As data processing systems grew more complex, to maintain the synchronization with reduced time for scanning a gap in high performance DASD limited the cable length between the peripheral subsystem and the host processor. The reason for the cable length limit is signal propagation times. Recently, the use of optical fibers in a so-called serial channel arrangement has been provided in which the burst data rate is substantially higher than of the previous channels, such as a four to one increase. Such serial channel data burst rate is also substantially higher than the data burst rate of present day DASDs. Further, the cable lengths of the serial channels are longer than the prior cable length limits resulting in substantially longer signal propagation times between the peripheral subsystem and host processors. Such cable lengths have prevented the above described synchronized operation between the peripheral subsystem and the host processor, i.e., the signal propagation times prevent effective interaction of the peripheral subsystem and the connected host processor while the transducer is scanning the gap between records or fields on the DASD.
For accommodating longer cable length signal delays in the synchronous mode, the gap lengths in DASD could be increased. Such longer gaps increase delay time between accessing successively scanned records and reduces storage capacity, both effects are undesired.
Another configuration of peripheral subsystems includes a large scale cache interposed between the DASD and the channel connected to the host processor. It is desired to provide efficiency-raising controls in a cached DASD subsystem while maintaining system and data integrity of the data processing system. The operation of such a cached DASD peripheral subsystem is in a so-called "non-synchronous mode" which means that operations between the peripheral subsystem and the host processor are not synchronized to operation of DASD, specifically no control interactions with a host processor are completed while the DASD transducer is scanning a gap. This arrangement of removing the synchronization has to be carefully controlled otherwise unwanted or unintended rotations of the disk between operations could be required. Such additional disk rotations reduce channel utilization and the performance of both the peripheral data subsystem and the entire data processing system. Therefore, it is desired to enhance the operation of such non-synchronous mode such that channel utilization is enhanced. It is to be remembered that each peripheral subsystem may share a serial cable or optical fiber to one or more host processors and that each host processor may be connected to a plurality of peripheral subsystems via the same cable or channel. It is desired to reduce the time each subsystem uses such channel connections for enhancing the channel utilization which thereby increases the productivity of the entire data processing system.